Porous ceramics, such as are produced by the replication process, are well known in the art. The replication process has been utilized widely. In its more basic form, replication of pores or holes includes the preparation of lightweight ceramic blanks molded with a combustible or volatile constituent material, for example. When fired at high temperatures, the combustibles burn away, leaving pores of the same shape in the surrounding ceramic. Such a replication technique has found particular application where porous ceramic, metal, fibrous and polymeric replicates are prepared for filtering chemicals, water, hydrocarbons, gases, and the like.
In filtering technology, it is known to coat organic structures, such as polyurethane foams, with a ceramic composition, as disclosed in U.S. Pat. Nos. 1,856,475, 3,214,265, 3,090,094 and by 3,097,930, for example.
U.S. Pat. No. 3,090,094 discloses the use of silicate and phosphate binders to aid in supporting the refractory ceramic particles in order to prevent the unsintered ceramic from collapsing during the organic burnout phase of the process. The examples disclose that such ceramics were fired to temperatures between 2100.degree.-2500.degree. F. It is well known to those familiar with the state of the art that such low process temperatures suggest that such compositions are less refractory than those requiring higher maturing temperatures.
U.S. Pat. No. 3,097,930 shows aluminum oxide (alumina), beryllium oxide (beryllia) and china clay as specific major ingredients of three separate porous ceramic structures. The porous alumina ceramic as taught by Example I of this patent is expected to be resistant to high temperatures, moderate in cost, but not thermal shock resistant. The porous beryllia ceramic as taught by Example II is also expected to be resistant to elevated temperatures, but very expensive due to the inherent cost of beryllium oxide. The porous china clay ceramic as taught by Example III has a low firing temperature of about 1350.degree. C., and consequently has a relatively low temperature resistance.
U.S. Pat. No. 3,898,917 discloses the preparation and application of such ceramic foam structures for filtering molten metal--particularly molten aluminum alloys, and copper and its alloys--where the melts are generally below 2000.degree. F.
U.S. Pat. No. 3,947,363 describes a ceramic composition containing aluminum oxide with chromium oxide, bentonite and a ceramic binder, preferably aluminum orthophosphate. The '363 patent discloses that other air setting binders can be used, but aluminum orthophosphate is preferred. This binder contributes to the softening of the resulting fired ceramic foams beginning at 2000.degree. F. or so. The inclusion of bentonite contributes silica, which along with the other constituents, results in a glassy phase that is subject to softening at temperatures beginning as low as about 1600.degree. F., depending upon the composition.
High purity aluminum oxide ceramic foams have been produced for filtering molten high temperature cobalt and nickel alloys containing hafnium as a minor ingredient. Such high purity filters are necessary since hafnium reacts with silica, phosphorus and boron, which may be present in less pure refractories, to thereby contaminate the resulting normally pure alloy casting. Other alloy ingredients, such as titanium, chromium and the like, also react with trace amounts of other minor elements such as may be found in impure refractories which may be contacted by such molten alloys, such as tundishes, pouring spouts, filters etc. The filtering of such alloys in particular requires ceramic filters in which contaminating or contaminate producing minor elements are eliminated or present only in specified limited amounts.
These ceramic foams ar therefore limited due to their low resistance to softening at temperatures of about 2000.degree. F. or less. Ceramic foams prepared with phosphate compounds form glassy phases which soften as they become heated above red heat. The addition of phosphates, borates and silicates to more refractory compounds are known to contribute to the formation of glassy phases which tend to become plastic as such products are exposed to high temperatures.
The filtering of cobalt and nickel high temperature alloys, as well as other less refractory alloys, often requires that the ceramic filters be highly resistant to thermal stress. This is in order that the molten alloys being cast at temperatures exceeding 2500.degree. F. do not cause the ceramic filters to shatter, and thereby contribute small ceramic particles to the resulting castings. Such contaminating ceramic particles are known to cause defects which lower the fatigue and stress resistance of the alloys in their intended application.
It is known that ceramics are more brittle than most metal and plastic materials. This is due to the inherent inability of ceramic materials to resist significant deflection, i.e., strain. Ceramists have discovered that solid ceramics can withstand more thermal strain when the major ceramic crystal grains include a great number of even smaller crystal grains of a second phase, such as other ceramic compounds. For example, fired dense aluminum oxide solids which contain many discrete fine particles of zirconium oxide are known to be more tough and thermal shock resistant than the same material without such scattered particles within the grain structure. The discrete zirconium oxide particles act as "crack stoppers", and inhibit the propogation of cracks in the principal aluminum oxide grains to short localized areas. This phenomenon was addressed by R. L. Cox in "Temperature Dependence of Transformation Toughening in Alumina-Zirconia Ceramics", on May 5, 1982, at the annual meeting of the American Ceramic Society.
Another reported mechanism for increasing the resistance of brittle ceramics to strain failure is by increasing ductility through diffusional creep. This phenomenon was addressed by Y. Ikumo and R. S. Gordon in "Diffusional Creep of Polycrystalline Alumina Doped with Manganese-Titanium and Iron-Titanium Impurity Pairs", presented on May 5, 1982, at the annual meeting of the American Ceramic Society.
There is a need in the industry for an inexpensive fired ceramic composition which has little or no glassy phase at temperatures in excess of 2000.degree. F., which includes few or no potentially melt contaminating minor ingredients and which, in particular, has an exceptional shock resistance.
It is therefore an object of this invention to provide an improved fired ceramic composition with high temperature resistance.
It is another object of this invention to provide a fired ceramic composition with improved thermal shock resistance.
It is a further object of this invention to provide shock resistant refractory ceramic structures with little or no glassy phase.
It is yet another object of this invention to provide a ceramic foam, such as for use as a filter, which has improved high temperature resistance and is resistant to degradation under severe conditions of use, such as in the filtering of molten metal.
It is a principal object of this invention to provide a ceramic foam which has improved resistance to thermal shock.
A further object of this invention is to provide such a ceramic foam with high compressive strength and with relatively low pressure drops.
Yet another object is to provide an improved ceramic foam filter useful for filtering aluminum and other low melting point alloys at temperatures upward of about 1000.degree. F., but particularly useful for filtering metals and alloys at molten metal temperatures above about 2500.degree. F.
It is a further object of the invention to provide a fired ceramic composition that can be exposed to rapid temperature changes while resisting fracture due to thermal stress.
It is still another object to produce such fired ceramic compositions and foams with the use of relatively inexpensive ingredients, and which are substantially nonreactive to molten metal alloys.